The present invention relates to a ball screw apparatus, and in particular a technique effective for realizing more improvement of low noises, operationability and endurance.
For example, the ball screw apparatus is, as shown in FIGS. 8 and 9, composed of a screw shaft 1 having a first spiral screw groove 3 on an outer circumferential face thereof, and a nut 2 having a second screw groove 4 on an inner circumferential face in opposition to the first screw groove 3 defined in the screw shaft 1 and being in mesh with the screw shaft 1 via a plurality of balls 5 rotatably charged in a spiral ball rolling path R defined between the first and second screw grooves 3, 4. The ball rolling path R is provided at its one end with a ball circulating path J for taking up the balls rolling and tracking in the ball rolling path R and sending to the other end. If relatively rotating the screw shaft land the nut 2, and moving one of them in an axial direction, this ball screw apparatus endlessly performs relatively spiral movement between the screw shaft 1 and the nut 2 through rolling of the plural balls 5.
In the ball screw apparatus of such a structure, the ball rolling path R has a face of, for example, Gothic arch, and the ball 5 rolls under an angular contact with the R-face of the ball rolling path at two to four points, while the ball rolling path R spirally continues and by having a return mechanism of the ball circulating path J, change from a non-loaded condition to a loaded condition happens. Since the ball 5 under the loaded condition (called as “loaded ball” hereafter) does not form an ideal rolling condition but creates a spin phenomenon, the ball is easy to cause slip and a ball abrasion occurs. Further, since the adjacent loaded balls 5 contact under a condition of rolling in opposite directions each other, slip is caused between the loaded balls 5. As a result, the loaded balls 5 are prevented from free rolling, and there arise various problems of bad working of the loaded balls 5, creation of abrasion and damage, fluctuation of torque, or occurrence of noises.
Particularly, in the ball screw apparatus effected on and off with high load applied to an injection molding machine or pressing operation, since osculating ellipse is large owing to elastic deformation between the ball 5 and the ball rolling path R, abrasion of the loaded ball is large by spin, and competition of the ball 5 particular to the ball screw apparatus occurs, so that the abrasion and damage appear more remarkably.
For solving these problems, as shown in FIG. 9, such a measure has been broadly practiced where the retaining pieces 10 having recesses 11 at both ends in the ball tracking direction are interposed between the adjacent balls 5 rolling and tracking in the ball rolling path R, and this measure enables to suppresses competition of the balls 5.
However, by interposing the retaining pieces 10, a new problem arises that the retaining piece 10 and the ball 5 compete against each other, so that the retaining pieces 10 are effected with abrasion and damage. Abrasion and damage of the retaining piece 10 create spaces around the retaining piece 10, and a sum total in spaces of the retaining pieces 10 and the balls 5 at an initial period (when setting the retaining piece 10 to the ball screw apparatus) inconveniently grow while using the ball screw apparatus.
Further, if the retaining piece 10 is formed with a resin, depending on sorts of resins (for example, Nylon based resins enabling mass-production by an injection molding at low cost), an expansion phenomenon occurs which absorbs water content and varies dimensions, so that contact points of the adjacent balls 5 are made near inconveniently. As a result, spaces are created around the respective retaining pieces 10, and therefore the sum total of the spaces between the retaining pieces 10 and the balls 5 is larger than that of an early condition.
Herein, when the sum total in spaces of the retaining pieces 10 and the balls 5 increases and the spaces gather to one position, the retaining piece 10 falls as shown in FIG. 10 to probably cause bad working or earlier damage of the ball screw apparatus.
Japanese Patent Laid Open 21018/2001 discloses therefore a measure of interposing elastic materials between the adjacent balls together with the retaining pieces. Depending on this measure, if the ball is effected with high load on and off, it is possible to suppress abrasion and damage acting on the retaining pieces and the balls.
Japanese Patent Laid Open 199556/2000 discloses a measure formed with ruggedness on contacting faces with the balls of the retaining pieces. According to this measure, the retaining pieces and the balls roll while point-contacting, thereby enabling to suppress abrasion and damage of both.
However, in the above Patent Laid Open 21018/2001, if the elastic material placed between the adjacent balls together with the retaining pieces is loaded, spring force of the elastic material acting on the loaded ball is powerless, and inconveniently a space adjusting capacity is not enough. Therefore, depending on space growth owing to abrasion and damage of the retaining piece and the loaded ball, or dimensional change of the retaining pieces by the expansion phenomenon, the retaining piece might fall, and there still remains a room for improvement of maintenance in the sum total of spaces between the retaining pieces and the balls or prevention from falling of the retaining pieces.